Apparatus for flash evaporating, filming and extruding viscoelastic material

ABSTRACT

Apparatus for evacuating viscoelastic fluids from a cylindrical vessel comprising rotating said fluid at the base of said vessel while, concurrently, directing said fluid toward and through an annular exit space around the peripheral internal walls of said vessel and exerting a downward force on said fluid contact said base of said vessel. A specific form of the invention may be employed for the devolatilization of polymeric materials.

United States Patent Inventor James R. Fisher Piacataway, NJ.

Appl. No. 823,928

Filed May 12, 1969 Patented July 27, 1971 Assignee Union CarbideCorporation New York, N.Y.

APPARATUS FOR FLASH EVAPORATING, FILMING AND EXTRUDING VISCOELASTlCMATERIAL 6 Claims, 3 Drawing Figs.

US. Cl ..L 159/2, 159/6, 159/13 1nt.Cl. B01d 1/28, 801d 1/22 Field ofSearch 159/6 W 13A,47,2E, 13, 13 A,5,7, 2; 18/12 SA, 12 SH: 12 SV, 12DR; 202/236 [56] References Cited UNlTED STATES PATENTS 3,211,20910/1965 Latinen et al 159/6 W 3,242,969 3/1966 Kiguchi 159/6 W 3,217,78311/1965 Rodenacker 159/2 E 3,361,537 H1968 Ferrante 23/283 PrimaryExaminer-Wilbur L. Bascomb, Jr.

Assistant ExaminerJ. Sofer Attorneys-Paul A. Rose, Gerald R. OBrien, Jr.and Aldo J.

Cozzi ABSTRACT: Apparatus for evacuating viscoelastic fluids from acylindrical vessel comprising rotating said fluid at the base of saidvessel while, concurrently, directing said fluid toward and through anannular exit space around the peripheral internal walls of said vesseland exerting a downward force on said fluid contact said base of saidvessel. A specific form of the invention may be employed for thedevolatilization of polymeric materials.

1 To vocuum H Drive meons PATENTEOJULZYISYL 3,595,296

ATTORNEY APPARATUS FOR FLASH EVAPORATING, FILMING AND EXTRUDINGVISCOELASTIC MATERIAL The present invention relates to improved methodand apthe like. Many methods and related apparatus have been suggestedfor evacuating viscoelastic fluids from cylindrical vessels. One suchmethod is disclosed and claimed in US. Pat. No. 3,395,746, issued onAug. 6, 1968 to T. T. Szabo et al. As there disclosed, foamy liquidpolymer composition is discharged from a cylindrical vessel, ,aftertreatment, by passage through a pair of counterrotating, intermeshingextruder screws. A throughput capacity limitation is inherent in the useof such means to evacuate vessels on a continuous, high-throughputbasis.

Other methods for accomplishing the evacuation of cylindrical vesselsare disclosed in U.S. Pat. Nos. 3,113,843 and 3,36l,537, both of whichemploy conical-shaped, ribbon-type extensions within the cylindricalvessel which feed material from the sidewalls of the vessel to a screwextruder. In addition to requiring a liquid level within the cylindricalvessel, the methods and apparatus of these patents present the sameinherent limitation as to throughput capacity for operation on acontinuous basis.

Accordingly, it is a prime object of the present invention to providemethod and apparatus for the effective evacuation of viscoelastic fluidsfrom cylindrical vessels, whereby continuous operation at highdhroughputcapacities can be achieved.

In accordance with one aspect of the present invention, a method isprovided for the evacuation of viscoelastic fluids from cylindricalvessels which comprises rotating said fluid at the base of said vesselwhile, concurrently, directing said fluid toward and through an annularexit space around the peripheral internal walls in the region of thebase of said vessel and exerting a downward force on said fluidcontacting said base of said vessel.

Apparatus suitable for employment in the practice of the method aspectof the invention isapparent in the following specification and drawings,in which:

FIG. 1 is a schematic, partial elevational sectional view of apparatusembodying the invention;

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. I; and

FIG. 3 is a cross-sectional view of modified apparatus, as would be seenif taken along the line 3-3 of FIG. 1 without the flow of the foamypolymer strand shown in FIG. 1.

Referring specifically to the drawings, a cylindrical vessel 10, in theillustrated case a devolatilization chamber, is provided having inletmeans 12 at the upper end thereof for introducing polymeric material andvacuum vent means 12a. A foamy strand 13 of polymeric material is formedby the introduction of liquid polymeric composition intodevolatilization chamber 10, which is maintained at a relatively reducedpressure in order to flash vaporize the volatiles and form the foamystrand. The base of devolatilization chamber comprises rotating member14 providing at its top rotating base surface 16. The diameter ofrotating base surface I6 is slightly less than the inner diameter of thewalls of the devolatilization chamber, thereby providing annular exitspace 18 between rotating member 14 and the walls of thedevolatilization chamber.

Baffle plate means 20 are provided, secured to the sidewalls 21 ofdevolatilization chamber 10. Baffle plate means 20 preferably comprisesa hemicylindrical tube section having a diameter of approximatelyone-half the inner diameter of the devolatilization chamber and ispreferably positioned so as to be radially mounted within thedevolatilization chamber with its curvature convexly opposed to thedirection of rotation of rotating base surface 16. A converging recessregion 22 is thereby provided between sidewalls 21 of devolatilizationchamber 10 and baffle plate means 20.

A relatively small, constant, predetermined clearance 23 is providedbetween the lower portion of baffle plate means 20 and rotating basesurface 16.

It has been found to be essential in the practice of the method andapparatus that downward forces be exerted on the viscoelastic fluidpositioned at the base of the cylindrical vessel to be evacuated. Thisis preferably accomplished by an inclination of the baffle plate means,in the direction opposite to the direction of rotation, from a positionnormal to the surface of the rotating base. The imposition of this forceand the resulting compression of the viscoelastic fluid at the base ofthe vessel causes the fluid to adhere to the rotating surface andproduces both a component of the pumping action which feeds the materialinto the converging recess region, as well as the action which causes aportion of the material behind it to pass through the predeterminedclearance between the baffle plate means by adhering to the surface ofthe rotating base.

Rotating member 14 is in the shape of an inverted truncated cone having,as mentioned hereinabove, its base surface positioned upwardly asrotating base surface 16 and its truncated conical surface 24 positioneddownwardly. A spiral, helical screw means 26 is positioned on conicalsurface 24. A conical annular passage 28 is provided between convergingbase walls 29 of the devolatilization chamber and truncated conicalsurface 24 which communicates with discharge port means 30 at the lowerend thereof.

Extruder means 32 is axially and integrally connected to andcommunicating at its inlet with a discharge port means 30. The extrudermeans 32 comprises an outer housing 32 and extruder screw 35 having aforward screw flight 36. Material is passed under compression throughthe extruder and discharged through discharge port means 38. A reversescrew flight 40 is provided, below extruder discharge port means 38, toassist in the discharge of material therefrom.

The shafted assembly of extruder screw 35 and rotating member 14 iscommonly driven by drive means 42.

In operation, polymeric material is flashed through inlet means 12 tothe devolatilization chamber 10 to release volatiles and form the foamystrand 13 (or multiple strands) which is preferably axially introducedinto the chamber as shown in the embodiments of FIGS. 1 and 2. Thefoamed material passes toward the base end of the devolatilizationchamber and collects behind the baffle plate means, as shown in FIG. 2of the drawings. The rotation of rotating base surface 16 may cause thebuildup of the foamed plastic material on that rotating base surface inthe pattern schematically shown in FIG. 2.

In operation of the devolatilizer, it has been found that the buildup ofmaterial behind the baffle is a function of the speed of rotation. Athigh speeds, i.e., approximately 92 r.p.m., it was found that no buildupof material was formed. The material was conveyed to the edge of therotor fast enough and at the high speeds the screw would remove thematerial fast enough such that no roll was formed. At speeds lower thanthis, i.e., around 40-50 r.p.m., a buildup was formed behind the baffle,and as rotor speed decreased the roll increased in size.

Rotation causes the feeding of material toward the outer periphery ofthe base surface and effects a pumping action of the material into theconverging recess region 22. At the same time, the material behind thatin the converging recess region 22 passes to the baffle and a sized thinfilm of material is passed through the predetermined clearance 23(between baffle plate means 20 and rotary base surface 16) tocontinuously provide the exposure of a new thin film of foamed plasticmaterial to the devolatilization chamber. This will assist in effectingmore complete devolatilization of the material when the method andapparatus are employed for devolatilization applications. It isimportant to note that, when the present invention is employed fordevolatilization applications, the apparatus should be operated so thatthere is no appreciable fluid level or stagnant areas where fluidmaterial can build up.

vessel.

In the modified embodiment of apparatus shown in FIG. 3 of the drawings,a full hemicylindrical plate baffle means is not employed, the baflleplate means there being a section or portion of hemicylindrical plate.In the case of this embodiment, it is to be understood that, where theapparatus is to be operated as a devolatilizer, the incoming foamystrand of polymeric material should, most preferably, be introducedaxially into the devolatilizing chamber, rather than introduced nearerthe outer periphery. This is so since it is desirable to keep the foamystrand equidistant from the chamber walls so 1 that, as the strand isblown about by escaping gaseous volatiles, it will not tend to adhere toone side of the chamber. This is to be avoided if reliable operation ofthe unit is desired.

If one were to introduce the foamy strand off center in the chamber,then the shortest distance to the chamber wall 1 would require a largerdiameter chamber than if the stream were introduced on the chamber axis.This would result in increased cost. lfaxial introduction is employed inthe apparatus of the FIG. 3 embodiment, it is necessary to rely oncentrifugal forces to assist in the passage of foamed polymeric materialfrom the center to the outer regions of the rotating base surface, inorder to enable it to reach the region of the baffle extruder hopper, ora bin containing solid particles or slurries), flow rate capabilitiesmay be the prime determinant of most preferred angular velocities ofrotation. In addition, where treatment of the material is being carriedout during this evacuation of viscoelastic fluid from the vessel (suchas devolatilization), a significantly different angular velocity ofrotation and consequent flow rate may be most preferable. In practice,angular velocities less than rpm. have been employed for someapplications, whereas angular velocities of the order of 100 rpm. andhigher have also been employed.

As specific examples of operations of the method of the presentinvention, streams of polymeric fluid were introduced into adevolatilization chamber of the type shown in the figures of thedrawings, having a height of 24.25 inches and an inside diameter of 6inches, in the form of a Biz-inch diameter of crystal polystyrene andethyl benzene. The baffle was inclined against the direction of rotationat an angle of approximately l8 from a perpendicular to the rotatingbase surface.

The polymeric fluid was fed to and evacuated from the chamber under thefollowing conditions for each of the seven examples listed in thefollowing table.

TABLE Discharge Inlet solvent solvent Flash Flow concentrationconcentration chamber rate of lb. solvent/lb. lb. solvent/1b. pressure,Inlet Discharge Rotor solution dry polymer, dry polymer, mm. Hg temp,temp speed, (inlet) percent percent absolute C. r.p.m lb./hr.

What I claim is:

plate means and produce the consequent pumping action necessary for theevacuation of material from the cylindrical It is believed that thetotal pumping action which effects evacuation of material from thecylindrical vessel through annular exit space 18 is contributed by thefollowing components: the confining action through direction andcompression into the converging recess region 22; the viscoelasticforces generated within the fluid material; and the compressive pumpingaction of the helical compression screw means 26 which receives thematerial from the annular exit space 18.

After passage of the foamy polymeric material into the conical annularpassage 28, it is preferably compressed by helical compression screwmeans 26 to effect further devolatilization. Therefore, the volatilesdriven off by compressive devolatilization in annular passage 28 passback through the foamy polymeric material mass to the devolatilizationchamber.

The volatiles are distributed quite slowly and uniformly in their returnthrough the mass of foamed polymeric material being treated. There is nobubble fonnation in or on the buildup of material formed behind thebaffle in the apparatus of the invention. The major portion of thevolatiles are removed from the polymeric material in the flash chamberproper as the foaming strand passes down through it. The additionaldevolatilization from surface regeneration, provided between baffle androtor surface and in the annular passage 28, is not a foamy-typedevolatilization, i.e., no large bubbles or cells are formed, sincethere are not enough volatiles remaining in the solution to create alarge number of bubbles.

The most desirable angular driving speeds to be employed may varyconsiderably, depending upon the viscosity and composition of theviscoelastic fluids being evacuated from the cylindrical chamber.Similarly, the most desirable speeds may vary widely, depending upon thetype of processing treatment being effected on the viscoelastic fluids.For example, where it is desired merely to evacuate viscoelastic fluidfrom a cylindrical vessel (such as an autoclave or other reactor, an

1. Apparatus for evacuating viscoelastic fluid from a generally verticalcylindrical vessel comprising: cylindrical vessel means having inletmeans near the upper end thereof, and a rotating member mounted in thebase thereof so as to provide a rotating base circular plane surface andan annular exit space around the peripheral internal walls of saidvessel, and baffle plate means secured to the sidewalls of said vesselin the vicinity of and relatively close clearance with said base andpositioned to form an acute dihedral angle with said vessel sidewalls onthe upstream side of the baffle with respect to the direction ofrotation of said base and to direct said fluid from said rotating basesurface to the point of said annular exit space around the peripheralinternal walls of said vessel which is the vertex formed by thesidewalls, baffle and base.

2. Apparatus in accordance with claim 1, wherein said baffle plate meansis inclined, opposite to the direction of rotation of said rotating basesurface, to apply a force to the fluid against said rotating basesurface.

3. Apparatus in accordance with claim 1, wherein said baffle plate meanscomprises a hemicylindrical tube section having a diameter approximatelyone-half the diameter of said cylindrical vessel means and positioned soas to be radially mounted within said vessel with its curvature convexlyand opposed to the direction of rotation of said rotating base surface.

4. Apparatus in accordance with claim 1, wherein said vessel comprises areduced pressure devolatilization chamber and said inlet means comprisesa confining passage through which liquid polymer material is dischargedinto said chamber and flashed to form a foamy strand of polymer.

5. Apparatus in accordance with claim 4, wherein said rotating meanscomprises an inverted conical body whose base surface comprises saidrotating base surface and whose conical surface carries helicalcompression screw means cooperating with the internal conical walls ofsaid vessel, for removing said fluid, under compression, from saidannular exit space to a discharge port positioned near the apex of saidinverted conical body.

vessel, with extruder means for further compressing said fluiddischarged from said vessel.

2. Apparatus in accordance with claim 1, wherein said baffle plate meansis inclined, opposite to the direction of rotation of said rotating basesurface, to apply a force to the fluid against said rotating basesurface.
 3. Apparatus in accordance with claim 1, wherein said baffleplate means comprises a hemicylindrical tube section having a diameterapproximately one-half the diameter of said cylindrical vessel means andpositioned so as to be radially mounted within said vessel with itscurvature convexly and opposed to the direction of rotation of saidrotating base surface.
 4. Apparatus in accordance with claim 1, whereinsaid vessel comprises a reduced pressure devolatilization chamber andsaid inlet means comprises a confining passage through which liquidpolymer material is dischargeD into said chamber and flashed to form afoamy strand of polymer.
 5. Apparatus in accordance with claim 4,wherein said rotating means comprises an inverted conical body whosebase surface comprises said rotating base surface and whose conicalsurface carries helical compression screw means cooperating with theinternal conical walls of said vessel, for removing said fluid, undercompression, from said annular exit space to a discharge port positionednear the apex of said inverted conical body.
 6. Apparatus in accordancewith claim 5, wherein said rotating means and the discharge portdefining portion of said vessel are axially and integrally connected,externally of said vessel, with extruder means for further compressingsaid fluid discharged from said vessel.